Control of an air-cooled heat exchanger

ABSTRACT

An air-cooled heat exchanger provided with a reversible pitch fan is controlled against excessive temperature swing in the material being cooled by the use of temperature-sensing elements and control circuitry which set the pitch of the fan for forward flow, reverse flow or feathering.

[72] Inventors United States Patent Paul E. Kline;

Charles E. Fahlgren; Merlin R. Kitchen, Midland, Mich.

[2!] Appl. No. 839,316

[22] Filed July 7, 1969 [45] Patented Feb. 23, 1971 [73] Assignee The Dow Chemical Company Midland, Mich.

' [54] CONTROL OF AN AlR-COOLED HEAT EXCHANGER 5 Claims, 2 Drawing Figs. l

[52] U.S.Cl 165/1; 165/39: 165/97; 165/122 [5]] Int. Cl B60h 1 /00 [50] Field otSearch 165/39, 97,

[56] References Cited UNITED STATES PATENTS 2,826,395 3/1958 Petty 165/39 Primary Examiner-Meyer Perlin Assistant Examiner P.D. Ferguson Attorneys- Griswold and Burdick. Richard G. Waterman and Robert B. Ingraham ABSTRACT: An air-cooled heat exchanger provided with a reversible pitch fan is controlled against excessive temperature swing in the material being cooled by the use of temperature-sensing elements and control circuitry which set the pitch of the fan for forward flow, reverse flow or feathering.

CONTROL OF AN AIR-COOLED HEAT EXCHANGER Oftentimes when employing air-cooled heat exchangers, particularly in the temperate or colder zones, difficulty occasionaliy is encountered by freezing of the process fluid being cooled or excessive cooling of the process fluid. Freezing of the process fluid within a heat exchanger which comprises a plurality of tubes or banks of tubes oftentimes results in substantial damage and disruption of the process. Thermal flexing of the heat exchanger may also lead to failure. Depending upon the relative temperature of the process fluid being cooled and the cooling air employed in a multilayer tubular heat exchanger, freezing may occur in a portion of the tubes adjacent the location where the air is entering or the ambient air temperature may drop sufficiently low that freezing may occur through the entire bank of tubes.

lt would be desirable if there were available an improved method and apparatus for control of air-cooled heat exchang devices. g

it would also be desirable if there were available an improved method and apparatus for the control ofair-cooled heat exchangers which would prevent freezing thereof while process fluid was being passed therethrough.

it would also be advantageous if there were available an improved and simple method of control of such air-cooled heat exchangers.

These benefits and other advantages in accordance with the method of the present invention are achieved in a method for controlling the temperature of a process fluid within an aircooled heat exchanger wherein air is forced through the heat exchanger by a reversible air forwarding means, the reversible air forwardingmeans being capable of assuming three conditions: (1) forwarding air into the heat exchanger; (2) withdrawing air from the heat exchanger, and (3) no air flow, the heat exchanger having a first or air intake end and a second or'air discharge end, the steps of the method comprising sensing the temperature of theprocess fluid adjacent the first end and adjacent the second end, selecting predetermined temperatures above the freezing point of the process fluid, passing air through the heat exchanger from one end to the other end when both the inlet and outlet temperatures are above the predetermined temperatures, causing air to flow is an air-cooled heat exhanger temperature control means comprising first and second temperature-sensing means adapted to be disposed at the inlet and outlet ends of an aircooled heat exchanger, respectively, an air-forwarding control means capable of at least three states: forward, reverse and off, means in cooperative combination with the temperaturesensing means and the control means to provide a signal permitting flow of air through the exchanger when first and "second sensing means are above a predetermined temperature, the means providing a flow of air from a warmer to a cooler end when temperature adjacent the cooler end is below a predetermined temperature and means to cause the air-forwarding means to be in the off condition when both temperatare-sensing elements indicate temperatures below predetermined values.

Further features and advantages of the present invention will become more apparent from the following specification taken in connection with the drawing wherein:

FIG. 1 schematically depicts control of an air-cooled heat exchanger in accordance with the invention.

FlG. 2 is an alternate embodiment of the invention.

In FIG. 1 there is schematically illustrated a heat exchanger and control assembly generally designated by the reference numeral Ill. The assembly 30 comprises in cooperative combination an air-cooled heat exchanger generally designated by the reference numeral ill. The heat exchanger 11 comprises a plurality of banks of heat exchange tubes 12 (only one tube of each bank shown). The tubes 12 are spaced from each other and the banks of tubes are spaced from each other to permit the passage of air therebetween. Each of the tubes 12 has an entrance end 13 and an exit or discharge end 14. The entrance end 13 is in operative communication with an inlet plenum 16 which in turn is in operative communication with a source 17 of heated process fluidr The discharge ends 14 of the tubes 12 are in operative communication with a discharge plenum 19. The discharge plenum 19 has an exit passageway 20 which discharges cooled process fluid from the heat exchanger 11. The heat exchanger 11 has an entrance or first end 21 and an exit or second end 22. An air-forwarding means 24 is disposed adjacent the first or entrance end 2!. The air-forwarding means or fan 24 is adapted for three states of operation: (1 forwarding air from the intake end to the discharge end; (2) drawing air from'the discharge end to the entrance end, and (3) an off or neutral position. As depicted in F l6. 1, the fan 24 has a reversible pitch propeller or fan 26which on application of suitable signal (in the form of gas pressure) will cause reverse flow, forward flow or no flow. Disposed within the tubes 12 adjacent the exit end21, is a first temperature-sensing means or thermocouple 29 which has associated therewith a reference junction 30. The thermocouple 29 and the reference junction 30 are in operative communication with an amplifier 31 adapted to increase the level of signal received from the thermocouple. The signal from the amplifier 31 is passed to a comparator or comparison amplifier 32. The comparator or differential amplifier 32 is also in operative combination with a source 33 of a stable reference signal or voltage. Beneficially, such a reference voltage is readily derived from a constant voltage source such as is obtained by means of a resistor and Zener diode or a variable source may be employed by placing a potentiometer across a Zener diode in a constant voltage reference circuit. The output of the comparator is fed to a control means or relay 34 which beneficially is a single-pole, double-throw relay. A generally similar thermocouple system is disposed within the tube 12 adjacent the entrance end of the heat exchanger 11. A thermocouple 29a is disposed within the tube l2 and is in cooperative combination with the reference junction 30a and signal from the reference junction 30a is passed to an amplifier 31a. The output of the amplifier 31a is passed to a comparator 320 which is in cooperative combination with an adjustable reference voltage source 330. The output of the comparator 32a is in operative combination with a switching means or relay 34a which, as illustrated, is a double-pole, double-throw relay. The switching control relay 34 is in operative communication with a source of control voltage 36 and the relay 34 alternately connects the source 36 to one of the two locations. These two locations are the poles of the relay 340. Three of the poles of the relay 34a provide a voltage to a first valve control means or solenoid 39 and a second valve control means or solenoid 40 or third valve control means 41. The valve control means 39, 40 and 41 are in operative combination with a first valve 39a, a second valve 3%, and a third valve 39c, respectively- The valve 390 provides selective communication with a first or A level source 42 of compressed gas, and a conduit 44 connecting with the variable pitch mechanism (not shown) of the fan 26. A bleed vent 45 is provided in the conduit 44 to permit reduction of pressure in the conduit 44 when a source of compressed gas such as the source 42 is disconnected therefrom. The valve 39a provides selective communication with the conduit 44 and a second source or B level source 46 of compressed gas. A valve 39c provides selective communication with a third source 47 of compressed gas such as compressed air at a level C. The fan or forwarding means 24 has three positions. When A level pressure is applied, the fan 26 forces air to flow from the first end 21 to the second end 22 of the heat exchanger 11. When B level air is applied to the fan 26, the pitch is reversed and air is drawn from the second end 22 to the entrance end 21. When C level air is applied, the fan 26 is feathered and no air is forced by the fan to move within the heat exchanger 11. Such mechanically controlled variable pitch mechanisms are well known, as well as hydraulic and electrically controlled variable pitch mechanisms.

In operation of the apparatus as depicted in FIG. 1, under normal conditions; that is, conditions where the ambient air does not cool the liquid below the predetennined temperature as set by the variable reference voltage source 33 and 33a, the relays 34 and 34a are in the unactuated position as shown in FIG. 1. In the unactuated position, the relays provide power to the actuator 39 maintaining the valve 39a in the open position and providing line 44 with air pressure at the A level. Air is drawn from the first end 21 to the second end 22. If the temperature as indicated by the thermocouple 29a falls below the predetermined temperature, a signal is provided to the relay 34a closing the relay and applying power to the actuator 40 and removing power from the actuator 39 thereby closing the valve 39a and opening the valve 39b and providing B level air to the line 44 causing the fan 26 to reverse its pitch and forward air from the second end 22 to the first end 21. If the temperature at the second end 21 rises above the predetermined temperature, A level air is reapplied and the pitch of the fan reversed. If for any reason the thermocouple 29 indicates a temperature below the predetermined temperature, the relay 34 is activated which also, through the relay 34a in its normally open position, opens the valve 3% and closes the valve 39a. If the temperature, as indicated by thermocouples 29 and 29a, is below the predetermined temperature, relays 34 and 340 both are actuated, applying power to the valve actuator 41 of the valve 390, resulting in the valves 39a and 39b being in the closed position and air at pressure level C is applied to line 44 causing the fan 26 to feather and cease to move air through the heat exchanger until such time as the temperature of the process fluid as indicated by one or both of the thermocouples 29 and 29a rises above the predetermined temperature and air is caused to flow in the appropriate direction. Opening of the relay 34 while the relay 34a remains closed results in B level pressure being applied through the valve 39b and reverse flow of air occurs. Whereas, if the relay 34a opens when the relay 34 is closed, air is forced to flow in the forward direction.

An alternate embodiment of the invention is depicted in FIG. 2 which shows only a control system and fan. A first double-pole, single-throw switch 55 is in operative communication with a temperature-sensitive actuator. The temperaturesensitive actuator may be of any variety including a bimetallic strip, a bellows, liquid filled bulb connected to an actuating diaphragm. The position of the switch 55 relative to the actuator 56 is controlled by a position adjustment means 57 which may comprise a cam and frame, screw thread and frame or the like. Such combinations of elements 55, 56 and 57 are well known and commercially available. A generally similar switch 55a of double-pole, singlethrow configuration is in operative communication with an actuator 56aand position adjustment means 570. An actuating power source 59 provides power to the common pole of the switch 55. The remaining poles of the switch 55 are connected to two of the poles of the switch 55a. One of the poles of the switch 55a in turn is operatively connected to an actuator such as an actuator 39b. The remaining pole of the switch 55a is connected to an actuator such as the actuator 40a. The actuators 39b and 40a actuate three way valves 39c and 39d in a manner substantially identical to the embodiment of FIG. 1. The valve 39d has in operative association with it a source of compressed gas 48a having a pressure level B. The valve 39d is in operative communication with a variable pitch fan 260 which operates in substantially the same manner as the fan 26 of FIG. 1. Thus, when the switches 55 and 55a are in the indicated position, the compressed gas to the pitch control of the fan 26ais vented to the atmosphere and has a pressure of zero. On closing of the switch 55a by a drop in temperature, gas of pressure B is supplied to the fan causing reversal of pitch and consequently direction of flow. When the switch 55 reverses its position and the switch 55a is closed, the fan is provided with gas of pressure A and is feathered.

Apparatus in accordance with the present invention is found eminently satisfactory for protecting air-cooled heat exchange units at ambient temperatures below 0 F. when employing water as the process liquid. Lower ambient temperatures have not been encountered as yet.

The embodiment set forth in FIG. 1 is particularly suited when the precise temperature control and maximum heat transfer are required. The heat exchanger may be mounted in any position; that is, vertical or horizontal air flow may be employed.

The arrangement of FIG. 2 is eminently suitable where greater temperature tolerance can be permitted and electromechanical switching is desirable and the heat exchanger is mounted in a vertical position (air flow upward or downward). Convection of warm air provides heat to the upper tubes if freezing occurs adjacent the upper end of the heat exchanger and the fan is feathered.

As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description and description.

We claim:

1. A method for controlling the temperature of a process fluid within an air-cooled heat exchanger wherein air is forced through the heat exchanger by a reversible air-forwarding means, the reversible air-forwarding means being capable of assuming three conditions: (I) forwarding air into the heat exchanger; (2) withdrawing air from the heat exchanger, and (3) no airflow, the heat exchanger having an air intake end and an air discharge end, the steps of the method comprising:

sensing the temperature of the process fluid adjacent the air intake end and adjacent the air outlet end;

selecting predetermined temperatures above the freezing point of the process fluid;

forwarding air through the heat exchanger when both the inlet and outlet temperatures are above predetermined temperatures;

causing air to flow from the outlet end to the inlet end when the temperature adjacent the inlet end is below a predetermined temperature; and

when the temperature at the inlet and outlet ends is below the predetermined temperatures, stopping flow of air through the heat exchanger by the forwarding means.

2. A method for controlling the temperature of an aircooled heat exchanger wherein the air-cooled heat exchanger for cooling a process fluid comprises a plurality of tubular heat exchange members in combination with inlet and outlet headers, an air-forwarding means comprising a motor-driven reversible pitch fan adapted to cause air to flow over the tubular members, the heat exchanger having an inlet end and an outlet end, the steps of the method comprising:

sensing the temperature of the process fluid at the inlet and outlet ends of the heat exchanger;

forwarding air from the inlet to the outlet end when the temperatures of the process fluid are above a predetermined temperature above the freezing point thereof;

when the temperature at the inlet end is below the predetermined temperature, reversing the pitch of the fan, thereby reversing the direction of airflow; and

when the temperature at the inlet and outlet ends is below the predetermined temperature, feathering the fan.

3. An air-cooled heat exchanger temperature control means comprising:

first and second temperature-sensing means adapted to be disposed at the inlet and outlet ends of an air-cooled heat exchanger, respectively, to sense the temperature of a process fluid flowing therein;

an air-forwarding control means capable of at least three speeds: forward, reverse and off;

means in cooperative combination with the temperaturesensing means and the control means to provide a signal permitting forward flow of air when first and second sensing means are above a predetermined temperature;

' and capable of being feathered;

means providing a reverse flow. when temperature adjacent the inlet end is below apredeten'nined temperature; and

means to cause the air-forwarding means to be in the off condition when both temperature-sensing means indicate temperatures below predetermined values.

4. An air-cooled heat exchanger comprising:

asecond header; v

process fluid-carrying tubes extending between and being in fluidcarrying cooperation with the-headers, the tubes having outlet ends in communication with an outlet header;

an air-forwarding means, the air-forwarding means comprising: t

a motor; and.

a fan, the fan adapted to force air through the heat exchanger from the heat exchanger inlet end to the heat exchanger outlet end, the fan being of reversible pitch a first temperature-sensing element in operative combination with the discharge end of the heat exchanger tube adjacent the inlet end, the temperature-sensing element being in cooperative combination with:

a signal-providing means, said signal-providing means adapted to cause reversal of the pitch of the fan;

a second temperature-sensing element disposed within the heat exchange tube associated with the outlet end of a heat exchange tube at the outlet end of the heat exchanger, the temperature-sensing means being in cooperative combination with:

a signaLproviding means which is adapted to feather the fan when the temperature is below the predetermined temperature as indicated by the first and second temperaturesensing elements. 9

5. The apparatus of claim 4 wherein the fan is pneumatically controlled. 

1. A method for controlling the temperature of a process fluid within an air-cooled heat exchanger wherein air is forced through the heat exchanger by a reversible air-forwarding means, the reversible air-forwarding means being capable of assuming three conditions: (1) forwarding air into the heat exchanger; (2) withdrawing air from the heat exchanger, and (3) no airflow, the heat exchanger having an air intake end and an air discharge end, the steps of the method comprising: sensing the temperature of the process fluid adjacent the air intake end and adjacent the air outlet end; selecting predetermined temperatures above the freezing point of the process fluid; forwarding air through the heat exchanger when both the inlet and outlet temperatures are above predetermined temperatures; causing air to flow from the outlet end to the inlet end when the temperature adjacent the inlet end is below a predetermined temperature; and when the temperature at the inlet and outlet ends is below the predetermined temperatures, stopping flow of air through the heat exchanger by the forwarding means.
 2. A method for controlling the temperature of an air-cooled heat exchanger wherein the air-cooled heat exchanger for cooling a process fluid comprises a plurality of tubular heat exchange members in combination with inlet and outlet headers, an air-forwarding means comprising a motor-driven reversible pitch fan adapted to cause air to flow over the tubular members, the heat exchanger having an inlet end and an outlet end, the steps of the method comprising: sensing the temperature of the process fluid at the inlet and outlet ends of the heat exchanger; forwarding air from the inlet to the outlet end when the temperatures of the process fluid are above a predetermined temperature above the freezing point thereof; when the temperature at the inlet end is below the predetermined temperature, reversing the pitch of the fan, thereby reversing the direction of airflow; and when the temperature at the inlet and outlet ends is below the predetermined temperature, feathering the fan.
 3. An air-cooled heat exchanger temperature control means comprising: first and second temperature-sensing means adapted to be disposed at the inlet and outlet ends of an air-cooled heat exchanger, respectively, to sense the temperature of a process fluid flowing therein; an air-forwarding control means capable of at least three speeds: forward, reverse and off; means in cooperative combination with the temperature-sensing means and the control means to provide a signal permitting forward flow of air when first and second sensing means are above a predetermined temperature; means providing a reverse flow when temperature adjacent the inlet end is below a predetermined temperature; and means to cause the air-forwarding means to be in the off condition when both temperature-sensing means indicate temperatures below predetermined values.
 4. An air-cooled heat exchanger comprising: a first header; a second header; process fluid-carrying tubes extending between and being in fluid-carrying cooperation with the headers, the tubes having outlet ends in communication with an outlet header; an air-forwarding means, the air-forwarding means comprising: a motor; and a fan, the fan adapted to force air through the heat exchanger from the heat exchanger inlet end to the heat exchanger outlet end, the fan being of reversible pitch and capable of being feathered; a first temperature-sensing element in operative combination with the discharge end of the heat exchanger tube adjacent the inlet end, the temperature-sensing element being in cooperative combination with: a signal-providing means, said signal-providing means adapted to cause reversal of the pitch of the fan; a second temperature-sensing element disposed within the heat exchange tube associated with the outlet end of a heat exchange tube at the outlet end of the heat exchanger, the temperature-sensing means being in cooperative combination with: a signal-providing means which is adapted to feather the fan when the temperature is below the predetermined temperature as indicated by the first and second temperature-sensing elements.
 5. The apparatus of claim 4 wherein the fan is pneumatically controlled. 